Communication systems are known to comprise a plurality of base sites that provide communication services to communication units located in corresponding service coverage areas of the base sites. One known communication system is a cellular communication system, such as the Global System for Mobile Communications (GSM). In the GSM system, a communication unit (e.g., a mobile or portable radiotelephone) that desires to communicate sends a channel request signal to a base site serving the coverage area in which the communication unit resides. Upon receiving the channel request signal, the serving base site allocates a communication resource for the communication. The communication resource comprises a coordinated pair of timeslots and frequencies (i.e., a first timeslot at an uplink frequency and a second timeslot at a downlink frequency). The timeslot at the uplink frequency supports transmissions from the communication unit to the serving base site; whereas, the timeslot at the downlink frequency supports transmissions from the serving base site to the communication unit.
Upon allocating the communication resource, the base site sends a channel designation signal to the communication unit via a common control channel. The channel designation signal contains the uplink timeslot and frequency, the downlink timeslot and frequency, and the transmit power for the communication unit. Upon receiving the channel designation signal, the communication unit tunes its transmitter and receiver to the designated timeslots and frequencies and begins communicating with a telephone network subscriber or another communication unit via the serving base site.
During a typical communication, the communication unit moves throughout the coverage area of the serving base site and the serving base site monitors a signal quality metric (e.g., received signal strength indication, RSSI) of the communication unit's uplink communication signal. To account for changes in RSSI as the communication unit moves, the serving base site will issue commands directing the communication unit to increase or decrease the communication unit's transmit power as the need arises.
Typically, as the communication unit moves to the periphery of the serving base site coverage area, the transmit power of the communication unit must be increased to account for an increase in path losses between the serving base site and the communication unit. Although the increased transmit power of the communication unit allows the serving base site to receive the communication unit's transmission, the increased transmit power also has the potential to interfere with an uplink communication signal at the same, or an adjacent, uplink frequency being used by a communication unit in another base site coverage area. That is, the higher power transmission of the communication unit can increase the cochannel or adjacent channel interference in the other coverage area as viewed by the base site serving that coverage area.
A known technique for mitigating the increased cochannel and adjacent channel interference introduced by an interfering communication unit is to reassign, or handoff, the communication unit that is being interfered with. This technique is described in detail in GSM Recommendation 05.08. Although this technique adequately mitigates the effects of the interference, it increases the handoff load of the system and fails to treat the cause of the interference (i.e., the interfering communication unit remains a potential interferer for the next communication unit that is allocated the previously interfered with uplink timeslot and frequency).
Therefore, a need exists for a method and apparatus for mitigating interference produced by a communication unit that eliminates the source of the of the interference without increasing the handoff load of the system.